1. Field of the Invention
The present invention concerns a method and system for reducing the duration of a phase for connecting a mobile to a base station in a cellular mobile radio network, typically in response to switching on or resetting the mobile.
2. Description of the Prior Art
In a cellular mobile radio network of the GSM (Global System for Mobile communications) or DCS-1800 type a mobile measures a receive level, typically a power level, for each frequency of the spectrum of the radio communication network concerned, for example in response to being switched on. In a GSM network there are 124 frequencies and in a DCS-1800 network there are 374 frequencies. A control frequency is associated with each cell or the corresponding base station of a network of this kind. This frequency is transmitted continuously in TDMA mode and conveys various data including synchronization data and network identification data, or PLMN data, in a particular time slot, known as the active time slot, typically time slot 0, in the downlink frame transmitted from a cell base station to the mobiles. Various networks co-exist within the same geographical area and constitute sub-networks of a common radio communication network such as a GSM radio communication network. A given user subscribes to one of these networks, known as the user's home PLMN, to use the GSM terminology. The various networks are managed by respective operators who are allocated particular frequency bands.
In a first prior art technique for connecting a mobile to a base station, for example in response to being switched on the mobile initially measures a receive level for each control frequency in the spectrum associated with the radio communication network in question; then, in decreasing received level order, it synchronizes to the active time slot to receive the PLMN data conveyed by the control frequency and thus identify the network of which the cell generating the control frequency is a part. The mobile carries out these operations in order to connect to a base station that meets both the following conditions: a) the station is part of the user's home PLMN, and b) it generates a control frequency received at a high level by the mobile.
In a second prior art technique described in "The GSM System for Mobile Communications" produced and published by M. MOULY and M. B. PAUTET, 1992 edition, pages 457-458--Cell Selection at Switch-On Time--and page 464, advantage is taken of the fact that a mobile switched off at a given location may be switched on again at the same location. Using this second technique, when the mobile is active and connected to a base station a list of control frequencies of the user's home PLMN transmitted by respective base stations in a geographical area containing the mobile is stored periodically in a non-volatile memory in the mobile. This non-volatile memory is typically a memory on the removable smart card SIM (Subscriber Identity Module) and the list of control frequencies is a list broadcast by the user's home PLMN. This list, transmitted on the control frequency of the base station to which the mobile is connected, indicates the control frequencies produced by the base stations of the network adjacent the base station to which the mobile is connected. Using this technique, during connection of the mobile, for example in response to being switched on the mobile carries out two successive procedures.
In the first procedure the mobile classifies the control frequencies in the list of control frequencies stored on the SIM card, in decreasing receive level order, using receive level measurements for each frequency of the spectrum carried out by the mobile when it is switched on. For each control frequency in the list of control frequencies, and in decreasing receive level order, the mobile synchronizes iteratively to each of them until it receives, in the data carried by each frequency from the list, a PLMN code identifying the user's home PLMN, in order to receive the PLMN data. This first procedure reduces the time to connect the mobile to the network if the mobile has not moved any significant distance between being switched off and switched on again. In this case the small number of control frequencies in the list, compared to the total number of control frequencies in the radio communication network, significantly reduces the time to connect the mobile. If the mobile has moved a significant distance, then its synchronization is imperfect and must be repeated. Synchronization is "perfect" if it is to the network base station nearest the mobile.
If the first procedure cannot connect the mobile to a base station of the user's home PLMN and generating a control frequency received at a sufficiently high level by the mobile, then the second procedure is carried out, and uses the first of the prior art techniques referred to above. For each control frequency in the spectrum of the radio communication network, and in decreasing received level order, the mobile synchronizes to the active time slot in order to receive the PLMN data and thus to identify the network of which the cell generating that control frequency is a part.
Accordingly, the reduced time to connect the mobile to a base station of the user's home PLMN resulting from the use of a shortened list of control frequencies selected from all the frequencies of the spectrum is effective only if the mobile has not moved significantly between being switched off and being switched on again. Otherwise, i.e. if the mobile has moved significantly, the duration of the first procedure based on the use of a shortened list of control frequencies, as described above, delays the connection of the mobile to a base station, compared to direct and immediate attempts to synchronize to all the frequencies of the spectrum, as in the first prior art technique.
The invention is directed to remedying this drawback by providing methods and systems for reducing the time to connect a mobile to a base station in a cellular mobile radio network, for example in response to switching on the mobile. These methods and systems reduce the time to connect a mobile to a network base station even if the mobile has moved between being switched off and being switched on again.